1. Field of the Invention
The present invention relates generally to cathode ray tube displays of the delta gun, shadow mask type. More particularly, the invention relates to improved convergence control apparatus for such systems.
2. Description of the Prior Art
Cathode ray tube (hereinafter CRT) full color displays, such as for example, those designed for displaying flight and navigation information to the pilots of an aircraft, require high resolution images which may be stroke written or raster written. In order to achieve such high resolution images, particularly those involving thin stroke written lines (alphanumerics, flight paths, target designators, etc.), superior convergence control is required.
As is well known to those skilled in the art of delta gun shadow mask type CRT's, each electron beam must precisely converge on a common hole in the shadow mask so that, upon exit therefrom, each beam strikes its corresponding color phosphor dot of the phosphor triea. Further, this convergence must be controlled with great precision not only when the beams are undeflected by the deflection coils, known as static convergence, but also throughout the full deflection range of the display, known as dynamic convergence. Also, as is well known in this type of tube, manufacturing tolerances in the assembly of the guns in the tube neck and possible stray magnetic fields in the display's environment, require adjustable magnetic fields adjacent the guns to achieve precise static convergence, while variable magnetic fields also adjacent the guns are required to assure precise convergence of the three beams as they are deflected by the deflection system across the entire face of the CRT. Dynamic convergence control is required due to the fact that most display screens are generally flat or only slightly convex relative to the point along the tube axis at which the beams are angularly deflected by the deflection coils, generally known as the pin cushion effect.
A number of techniques have been employed in the past for achieving static and dynamic convergence. Static convergence has been accomplished by employing manually adjustable permanent magnets supported on the tube neck to produce a magnetic field of adjustable direction and magnitude to cause the three electron beams to precisely converge at the center of the shadow mask. Both static and dynamic convergence have been achieved with some success in the past using a common electromagnetic convergence yoke fixed on the neck of the CRT which cooperates with internal pole pieces downstream from each of the three cathodes or electron guns. Controllable magnetic fields are produced across these internal pole pieces by controlling electrical current to the windings of each electromagnets as a function of the position of the beams as they are deflected by the deflection yoke horizontally and vertically across the tube screen.
The convergence control currents supplied to the convergence yoke coils have been generated in the past by a number of techniques. In some cases, these currents have been generated by wholly passive pulse shaping circuits using diodes, resistors, capacitors, inductances, etc. to produce the desired generally parabolic wave shapes. However, with such passive techniques, there is no independent control of the precise wave shapes for the individual electron guns. Furthermore, there is little or no control for convergence at specific points on the CRT screen. Additionally, the convergence signals generated interact with each other so that adjustment on one side of the screen can affect convergence on the opposite side. Procedures to achieve convergence using passive techniques require an interactive approach which results in a best achievable over-all convergence compromise making adjustment difficult and tedious.
Another convergence control technique used in the past employs digital rather than analog apparatus. With this technique, the screen is divided into a planar matrix of blocks, the total number of which depend upon the desired resolution or picture quality. The conversion currents required for each block of this matrix is stored in digital format in read only memories which are addressed as a function of beam position to read out the required conversion currents. Of course, the memory outputs require digital and analog conversion, suitable resistive mixing in the proper proportions to produce each of the three convergence signals for driving the coils of the convergence yoke electromagnets. While the digital convergence technique permits independent adjustment of each color and independent adjustment of convergence in each screen quadrant, it does have certain drawbacks. The resolution of the convergence correction is dependent upon the number of blocks in the screen matrix which in turn is dependent upon the size of the digital memories; the larger the number of blocks, the larger the memories and hence the ultimate cost. Also, the requirement for digital processing, such as A/D's, and D/A's further increases costs. Another drawback is the granularity typical of digital systems. For example, as the beams are deflected into the corners of the screen, the change in the required correction voltage from one block to an adjacent block becomes larger creating discontinuities or steps in the waveforms upon D/A conversion and objectionable patterns of faint lines in the corner regions of the screen.
A further technique used in the more recent past uses an active conversion correction technique. Basically, this type of convergence correction replaces both the digital technique and the passive amplitude-tilt technique of matching the well known parabolic waveforms to the screen with a technique that time divides each of the parabolic convergence waveforms vertical and horizontal into two halves thereby permitting adjustment of each half of the waveforms independently to achieve each of the required parabolic waveshapes across the tube face. In the known active techniques, the horizontal and vertical deflection sawtooth waveforms are filtered to remove undesired frequency components and then supplied to integrating circuits which serve to shape the input waves into appropriate parabolic waves. After proper shaping has been achieved, the waveforms are split in half and separately amplitude adjusted and recombined to form the vertical and horizontal components of the red, green and blud convergence signals. Additionally, corner convergence signals are generated by passive components means from the vertical sawtooth waveform and the horizontal parabola and combined with the horizontal convergence signal to produce the corner corrected horizontal convergence signals for the red and green beams. The vertical and horizontal convergence components are then fed to two single ended Class A transistor amplifiers which drive two separate windings (one for vertical and one horizontal) on each convergence yoke. The vertical and horizontal convergence component are added by the magnetic fields of the yoke to produce the complete red, green and blue convergence signals necessary to converge the beams.
The active conversion correction technique suffers from several drawbacks. The means for generating the parabolic waves from deflection sawtooths relies on passive integrators which are frequency sensitive and with work only on raster scan systems. Stroke writing displays have random deflection input waveforms making passive integrators unusable. Corner correction is also done with passive component waveshaping making it unusable in stroke writing displays. These systems employ single ended Class A transistor amplifiers to drive the convergence yoke windings. This type of yoke drive has several disadvantages. Amplifier current bandwidth is difficult to control. Current bandwidth of the convergence yoke driver must be approximately the same as the bandwidth of the deflection amplifier in stroke written displays. Heating in the coil windings cause the resistance of the windings to increase and will reduce the current through the winding causing a convergence error.
The convergence correction control apparatus of the present invention overcomes the difficulties of the prior art techniques. It provides high resolution, precision adjustment of all the parameters involved and requires only analog circuits thereby maximizing over-all accuracy and resolution and minimizing over-all costs.
Conversion correction yokes used in the past have suffered from electrical and mechanical complexity and delicacy. For example, such yokes utilize two convergence coils per electron gun, one for vertical correction and one for horizontal correction. Some used a single coil driver by the sum of the vertical and horizontal correction signals. Mechanically, both designs consist of a U-shaped ferrite core structure with windings on each leg. An air gap is cut in the center of the core and a wheel-shaped permanent magnet is placed over the gap. The resulting structure is mechanically delicate and difficult to package with the CRT and subject to shifting, particularly in a high vibration environment. Further, such yokes have relatively low sensitivities necessitating larger driving currents. The present invention provides a greatly improved convergence correction yoke design that provide high electrical sensitivity but is also simple and rugged mechanically.